The present invention relates to amorphous hard carbon films and methods for preparation the same. More in particular, the present invention relates to amorphous hard carbon films which exhibit good adhesion properties to a substrate and have improved tribological properties, wear resistance and friction coefficient, as well as methods for preparation the same. In addition, the present invention relates to mechanical parts having a surface or surfaces coated with the above amorphous hard carbon film.
Amorphous hard carbon films formed by vapor deposition methods such as CVD methods using a plasma or an ion beam are noted as hard coating materials because of high hardness (Vickers hardness: about 2000-3000). The amorphous hard carbon films are called as amorphous carbon films, diamond-like carbon films, i-carbon films or a-C:H films and are hard carbon films primary comprising amorphous carbon.
The amorphous hard carbon films do not adhere to a substrate depending on the kind of the substrate. Thus various improved methods have been proposed.
For example, Japanese Patent Un-examined Publication (JP-A-) No. 126972/1983 discloses that adhesion properties are improved by providing an intermediate layer between a substrate and an amorphous hard carbon film. However, the formation of the intermediate layer disadvantageously makes a film forming method tedious.
Japanese Patent Un-examined Publication (JP-A-) No. 300287/1992 discloses a method in which adhesive properties are improved by controlling the hydrogen content in an amorphous hard carbon film. The film obtained by this method has not sufficient adhesiveness to a substrate when it is used as coatings of mechanical parts.
Japanese Patent Un-examined Publication (JP-A-) No. 157602/1987 discloses amorphous hard carbon films containing metal elements such as silicon or the like. These films exhibit sufficient properties when they are used as coatings of mechanical parts. That is, the friction coefficient of the film is reduced by the addition of silicon, so that the slidable properties are improved. However, this film is unsuitable for the use as coatings of mechanical parts that are subjected to wear for a long period of time because of the reduced wear resistance and the increased wear.
In order to improve the wear resistance, it is assumed that an amorphous hard carbon film with a surface layer which is free from silicon would be preferred, and such a film is obtainable by a method in which the silicon content is gradually reduced from the bottom to the top of the layer by varying the composition of starting materials. However, the above procedures would be tedious and the adhesion properties would be reduced because of the increased internal stress in the film.
An object of this invention is to provide amorphous hard carbon films which exhibit good adhesion properties to a substrate and have improved tribological properties such as wear resistance and friction coefficient.
A further object of this invention is to provide a method for preparation of the above amorphous hard carbon films.
Another object of this invention is to provide mechanical parts in which a slidable member(s) is coated with the amorphous hard carbon film which exhibits good adhesion properties to a substrate and has improved tribological properties such as wear resistance and friction coefficient.